A substantial body of data indicates that the secondary hyperparathyroidism which develops during vitamin D-deficiency is responsible in part for the decreased renal response to parathyroid hormone. Experiments in this laboratory provided indirect information supporting the concept that vitamin D (metabolites) may be directly involved in regulation of the renal adenylate cyclase. (1) Thyroparathyroidectomy of vitamin D deficient rats resulted in a slow and only partial restoration of kidney parathyroid hormone dependent adenylate cyclase activity whereas TPTX of rats fed a calcium-deficient diet led to rapid and complete restoration of enzyme activity. (2) Normalization of serum calcium and parathyroid hormone in rats fed a D-deficient diet supplemented with calcium was associated with a significant decrease in renal PTH-dependent adenylate cyclase activity compared to control. (3) We have repeatedly found that repletion of D-deficient rats with either 1,25(OH)2D or 24,25(OH)2D did not restore the renal cyclic AMP response to parathyroid hormone to the control state even though serum calcium and parathyroid hormone were both normalized, whereas either vitamin D3 or 25-OHD was fully effective in this regard. These data provide the basis for the working hypothesis that: Vitamin D metabolites exert a specific direct interaction with parathyroid hormone in modulating the renal adenylate cyclase system of kidney. Experiments are designed to evaluate the direct effects of specific vitamin D metabolites as regulators of the renal PTH-dependent adenylate cyclase system using both in vivo and in vitro approaches. Effects of specific D-metabolites will be assessed in vitamin D-deficient, TPTX rats in vivo. Acute deficiency in 1,25(OH)2D produced by feeding 2.4% strontium will be examined to investigate whether 1,25(OH)2D is required for the proper response of adenylate cyclase to PTH. The in vitro experiments utilize a primary culture of kidney epithelial cells isolated from vitamin D deficient, neonatal rats. Direct effects of specific D-metabolites and combination of D-metabolites on the adenylate cyclase will be assessed in the proposed experiments. Both the receptors for parathyroid hormone and catalytic subunit activity will be investigated as possible molecular sites of regulation by vitamin D metabolites.